FILTERING RATE INHIBITION OF DAPHMA PULEX IN WINTERGREEN LAKE WATER1y2

The difference between the effects of seston concentration and dissolved substances on the rates of filtering and feeding by Daphnia pulex in the water of a small eutrophic lake was studied during late summer ( 16-18 August 1971). Low concentrations of 32P-labeled yeast, Rhodolorula sp., were added to whole lake water, lake water filtrate, and seston resuspended in filtered tapwater to measure grazing by field-collected animals. Lake water seston limited grazing rates at ambient concentration; “dis-solved ” inhibitors capable of limiting filtering rates at lower seston concentrations wcrc also present, but their effects were masked by high seston concentration. Possibly these dissolved substances were excreted into solution by the dense phytoplankton population, notably including Anabaena and Anacystis. Mathematical expressions for filtering and feeding rates as functions of seston concen-tration are derived from Holling’s disc equation and fit to the data. Rigler ( 1961) used 32P-labclcd yeast to collect data on filtering rate and food con-centration for Daplznia magna. He con-cludcd that his own results and those of Ryther ( 1954) and Marshall and Orr ( 1955) indicated a general phcnomcnon among fil-ter feeders: below a certain food concen-tration-later termed the “incipient limit-ing concentration ” ( cf. Fry 1947)-feeding rate is limited by the animal’s ability to filter water; hence feeding rate is directly proportional to food concentration and fil-tering rate is constant and maximum. Above the incipient limiting concentration, fecd-ing rate is limited by ingestion or digestion rate; hence feeding rate is constant and maximum, and filtering rate declines to an asymptote near zero. Reeve (1963a) found this same relationship between graz-ing rate and food concentration for Artemia salina. McMahon and Rigler ( 1965) idcn-tified this as the relationship to be ex